Is this incrementAndGet thread-safe? It seems to pull object from eh cache

Question

This servlet seems to fetch an object from ehCache, from an Element which has this object: http://code.google.com/p/adwhirl/source/browse/src/obj/HitObject.java?repo=servers-mobile

It then goes on to increment the counter which is an atomic long:

http://code.google.com/p/adwhirl/source/browse/src/servlet/MetricsServlet.java?repo=servers-mobile#174

    //Atomically record the hit
    if(i_hitType == AdWhirlUtil.HITTYPE.IMPRESSION.ordinal()) {
        ho.impressions.incrementAndGet();
    }
    else {
        ho.clicks.incrementAndGet();
    }

This doesn't seem thread-safe to me as multiple threads could be fetching from the cache and if both increment at the same time you might loose a click/impression count.

Do you agree that this is not thread-safe?

Answer 1

AtomicLong and AtomicInteger use a CAS internally -- compare and set (or compare-and-swap). The idea is that you tell the CAS two things: the value you expect the long/int to have, and the value you want to update it to. If the long/int has the value you say it should have, the CAS will atomically make the update and return true ; otherwise, it won't make the update, and it'll return false . Many modern chips support CAS very efficiently at the machine-code level; if the JVM is running in an environment that doesn't have a CAS, it can use mutexes (what Java calls synchronization) to implement the CAS. Regardless, once you have a CAS, you can safely implement an atomic increment via this logic (in pseudocode):

long incrementAndGet(atomicLong, byIncrement)
    do
        oldValue = atomicLong.get()            // 1
        newValue = oldValue + byIncrement
    while ! atomicLong.cas(oldValue, newValue) // 2
    return newValue

If another thread has come in and does its own increment between lines // 1 and // 2 , the CAS will fail and the loop will try again. Otherwise, the CAS will succeed.

There's a gamble in this kind of approach: if there's low contention, a CAS is faster than a synchronized block isn't as likely to cause a thread context switch. But if there's a lot of contention, some threads are going to have to go through multiple loop iterations per increment, which obviously amounts to wasted work. Generally speaking, the incrementAndGet is going to be faster under most common loads.

Answer 2

The increment is thread safe since AtomicInteger and family guarantee that. But there is a problem with the insertion and fetching from the cache, where two (or more) HitObject could be created and inserted. That would cause potentially losing some hits on the first time this HitObject is accessed. As @denis.solonenko has pointed, there is already a TODO in the code to fix this.

However I'd like to point out that this code only suffers from concurrency on first accessing a given HitObject. Once you have the HitObject in the cache (and there are no more threads creating or inserting the HitObject) then this code is perfectly thread-safe. So this is only a very limited concurrency problem, and probably that's the reason they have not yet fixed it.